High speed ground transportation systems with magnetically levitated vehicles are well known. At the beginning of the century, Graemiger proposed a levitation system based on magnetic attraction between an electromagnet in the vehicle and an overhead steel rail.
Bachelet proposed a system wherein the vehicle was suspended by repulsion generated by the interaction of magnetic fields excited in a coil mounted on the vehicle and fields generated by currents induced in a conducting plate. In order to reduce losses, passive loops or coils were substituted for the conducting sheet.
To better understand the prior art and the improvement of this invention, reference is made to FIG. 1, where a prior art levitated train 11 is illustrated traveling along a track or guideway which includes a conductive sheet 12. Eddy currents are induced in the sheet by the magnetic fields generated by the on-board coils. For purposes of illustration, a single coil 13 is shown mounted within the train 11. The interaction between the fields generated by the coils in the moving train and the fields generated by induced eddy currents in the conducting sheet 12 produce a repulsion or lifting or levitational force F.sub.L.
The force, F.sub.L, depends on the magnetic field generated in the coil and on the repulsion magnetic fields in the guideway or track. The magnetic fields in the guideway 12 are dependent on the speed of the vehicle since the induced currents are dependent on changes in flux. At higher velocity the changes in flux increase, thereby increasing the generated current and providing stronger magnetic fields and repulsion forces. The total force generated to levitate the train is dependent upon the number of coils and the train's speed. The conducting sheet has high resistive losses which can be considered a drag force, shown by F.sub.D. It had been proposed to substitute individually spaced low resistance coils as replacement for the conducting sheet to thereby lower the resistance and decrease the losses.
As the train speed is increased and the induced currents increase, the repulsion forces increase. Mechanical and electrical means have been provided for limiting the lift. Recently there has been proposed an elevated train with an on-board superconducting coil for generating the magnetic fields that interact with coils along the guideway.
Prior art electromagnetic suspension and propulsion systems present various problems. First, the trains must move at a predetermined speed or velocity before the induced currents generate fields which are of sufficient magnitude to suspend the train. Initially the train travels on wheels, and as it gains speeds it induces sufficient currents which provide repulsion magnetic fields which levitate the train. The magnetic fields for levitation are usually very high. Expensive and heavy shields are required to shield the passengers from the fields. In systems using superconducting coils, the weight, complexity and cost of the on-board refrigeration systems becomes relatively high. Because of the inherent instability due to changes in F.sub.L with speed, expensive control systems are needed to improve the dynamic stability.